Lecture 5 OIB Arc print 2008Ocean islands and seamounts Commonly associated with “hot spots”...
Transcript of Lecture 5 OIB Arc print 2008Ocean islands and seamounts Commonly associated with “hot spots”...
MORBPetrogenesis
Ocean islands and seamounts Commonly associated with “hot spots”
After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165-193.
TypesofOIBMagmasTwoprincipalmagmaseries
• Tholeii:cseries(dominanttype)– Parentaloceanislandtholeii:cbasalt,orOIT– SimilartoMORB,butsomedis:nctchemicalandmineralogicaldifferences
• Alkalineseries(subordinate)– Parentaloceanislandalkalinebasalt,orOIA– Twoprincipalalkalinesub‐series• silicaundersaturated• slightlysilicaoversaturated(lesscommonseries)
Evolu:onintheSeriesTholeii:c,alkaline,andhighlyalkaline
After Wilson (1989) Igneous Petrogenesis. Kluwer.
TraceElements:REEs
After Wilson (1989) Igneous Petrogenesis. Kluwer.
La/Yb (REE slope) correlates with the degree of silica undersaturation in OIBs
Highly undersaturated magmas: La/Yb > 30 OIA: closer to 12 OIT: ~ 4 (+) slopes → E-MORB and all OIBs ≠ N-MORB
(-) slope and appear to originate in the lower enriched mantle
MORB‐normalizedSpiderDiagrams
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).
TraceElements
• TheLILtraceelements(K,Rb,Cs,Ba,Pb2+andSr)areincompa:bleandareallenrichedinOIBmagmaswithrespecttoMORBs
• HFSelements(Th,U,Ce,Zr,Hf,Nb,Ta,andTi)arealsoincompa:ble,andareenrichedinOIBs>MORBs
IsotopeGeochemistry
• Isotopesdonotfrac:onateduringpar:almel:ngoffrac:onalmel:ngprocesses,sowillreflectthecharacteris:csofthesource
• OIBs,whichsampleagreatexpanseofoceanicmantleinplaceswherecrustalcontamina:onisminimal,provideincomparableevidenceastothenatureofthemantle
SimpleMixingModelsBinary
Allanalysesfallbetweentworeservoirs
asmagmasmix
Figure 14-5. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Sr‐NdIsotopes
Data from Ito et al. (1987) Chemical Geology, 62, 157-176; and LeRoex et al. (1983) J. Petrol., 24, 267-318.
After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
µ
MantleReservoirs
1.DM(DepletedMantle)=N‐MORBsource
After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
3.EMI=enrichedmantletypeIhaslower87Sr/86Sr(nearprimordial)
4.EMII=enrichedmantletypeIIhashigher87Sr/86Sr(>0.720,wellaboveanyreasonablemantlesources
After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
5. PREMA (PREvalent MAntle)
After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
EM and HIMU from crustal sources (subducted OC + CC seds) Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).
Distinctive Hot Spot Track Hawaii-Emperor Seamount
After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165-193.
HawaiianScenarioCyclic,pacerntotheerup:vehistory
1.Pre‐shield‐buildingstagesomewhatalkalineandvariable
2.Shield‐buildingstagebeginswithtremendousoutpouringsoftholeii:cbasalts
IslandArcMagma:sm Ac:vityalongarcuatevolcanicislandchainsalongsubduc:onzones
Dis:nctlydifferentfromthemainlybasal:cprovincesthusfar Composi:onmorediverseandsilicic Basaltgenerallyoccursinsubordinate
quan::es Alsomoreexplosivethanthequiescent
basalts Strato‐volcanoesarethemostcommon
volcaniclandform
• Igneousac:vityisrelatedtoconvergentplatesitua:onsthatresultinthesubduc:onofoneplatebeneathanother
• Theini:alpetrologicmodel:– Oceaniccrustispar:allymelted– Meltsrisethroughtheoverridingplatetoformvolcanoesjustbehindtheleadingplateedge– Unlimitedsupplyofoceaniccrusttomelt
Ocean‐ocean→IslandArc(IA)Ocean‐con:nent→Con:nentalArcor
Ac:veCon:nentalMargin(ACM)
Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding plate. PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.
StructureofanIslandArc
Schematic cross section through a typical island arc after Gill (1981), Orogenic Andesites and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6
joules/cm2/sec)
MajorElementsandMagmaSeries
• Tholeii:c(MORB,OIT)
• Alkaline(OIA)• Calc‐Alkaline(~restrictedtoSZ)
• MORB‐normalizedSpiderdiagrams– IntraplateOIBhastypicalhump
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.
MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Pearce (1983) with LIL on the left and HFS on the right and compatibility increasing outward from Ba-Th. Data from BVTP. Composite OIB from Fig 14-3 in yellow.
• MORB‐normalizedSpiderdiagrams– IA:decoupledHFS‐LIL(LILarehydrophilic)
What is it about subduction zone setting that causes fluid-assisted enrichment?
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.
Isotopes
Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures 13-11 and 10-15. After Wilson (1989), Arculus and Powell (1986), Gill (1981), and McCulloch et al. (1994). Atlantic sediment data from White et al. (1985).
Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).
Of the many variables that can affect the isotherms in subduction zone systems, the main ones are: 1) the rate of subduction 2) the age of the subduction zone 3) the age of the subducting slab 4) the extent to which the subducting slab induces
flow in the mantle wedge Other factors, such as:
dip of the slab frictional heating endothermic metamorphic reactions metamorphic fluid flow
are now thought to play only a minor role
yellow curves = mantle flow
Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).
P-T-t paths for the subducted crust in a variety of arc scenarios numerically modeled by Peacock (1990, 1991)
All curves are based on a subduction rate of 3 cm/yr, so the length of each curve represents about 15 Ma
Subducted Crust
Subducted crust pressure-temperature-time (P-T-t) paths for various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc) assuming a subduction rate of 3 cm/yr (Peacock, 1991, Phil. Trans. Roy. Soc. London, 335, 341-353).
2. The mantle wedge between the slab and the arc crust 3. The arc crust 4. The lithospheric mantle of the subducting plate 5. The asthenosphere beneath the slab
Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).
P-T-t paths for the subducted crust in a variety of arc scenarios numerically modeled by Peacock (1990, 1991)
All curves are based on a subduction rate of 3 cm/yr, so the length of each curve represents about 15 Ma
Subducted Crust
Subducted crust pressure-temperature-time (P-T-t) paths for various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc) assuming a subduction rate of 3 cm/yr (Peacock, 1991, Phil. Trans. Roy. Soc. London, 335, 341-353).
Now add the solidi for dry and water-saturated melting of basalt
Subducted crust pressure-temperature-time (P-T-t) paths for various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc) assuming a subduction rate of 3 cm/yr (Peacock, 1991). Included are some pertinent reaction curves, including the wet and dry basalt solidi (Figure 7-20), the dehydration of hornblende (Lambert and Wyllie, 1968, 1970, 1972), chlorite + quartz (Delaney and Helgeson, 1978). Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Subducted Crust Subducted Crust
A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.